Of the known methods and tools used for creating or subsequent processing of a thread or screw thread, some operate by cutting away some of the workpiece material in the region of the thread turns, whereas others do not remove material. The latter are based on deformation of the workpiece, producing the thread turns therein by exerting pressure. A survey of the thread-producing tools and processing methods in current use can be found in the Handbuch der Gewindetechnik and Frästechnik, editor: EMUGE-FRANKEN, publisher: Publicis Corporate Publishing, year of publication 2004 (ISBN 3-89578-232-7), which in the following is referred to simply as “EMUGE-Handbuch”.
The material-removing tools for thread production include thread tappers (cf. EMUGE-Handbuch, Chapter 8, pages 181 to 298) and milling cutters (cf. EMUGE-Handbuch, Chapter 10, pages 325 to 372) as well as, only for external threads, the threading die (cf. EMUGE-Handbuch, Chapter 11, pages 373 to 404).
A thread tapper is a thread-cutting tool that operates axially, with respect to the axis of the tool, with cutting edges disposed externally in a screw-shaped arrangement that depends on the pitch of the thread to be produced. During production of the thread the tapper is pushed forward axially into a bore in the workpiece, while rotating about the tool axis with a rotational velocity that depends on the forward velocity; in this process its cutting edges are permanently engaged with the workpiece at the wall of the bore (continuous cutting).
In the case of a thread milling cutter, several profiled ridges are offset from one another axially, with respect to the tool axis, in accordance with the pitch of the thread to be produced, but are not themselves oriented according to the thread pitch; these ridges are interrupted along the circumference by channels so as to form several cutting edges. To produce the screw thread the milling cutter is moved linearly forward along its tool axis while simultaneously making a circular movement about a central axis of the thread to be produced, or of the bore previously prepared in the workpiece, which results in a screw-like movement of the tool, the pitch of which corresponds to the pitch of the screw thread to be produced. In addition, the milling cutter is rotated about its own tool axis; the velocity of this rotation can be chosen from a broad range of values, so that it is independent of the velocity along the path of the screwing movement but is customarily distinctly higher than the latter. The cutting edges of the milling cutter engage the material of the workpiece intermittently, one after another (interrupted cutting). Thus the milling cutter is a circularly operating threading tool.
Among the thread-producing tools that do not remove material are thread forming tools such as the so-called thread-grooving tools sometimes also called thread rolling taps (cf. EMUGE-Handbuch, Chapter 9, pages 299 to 324) and thread rollers used only for external threads (cf. EMUGE-Handbuch, Chapter 11, pages 373 to 404).
Thread-grooving tools are threading tools that operate in an axial direction, with respect to the tool axis, by way of a region that forms a helix around the tool axis and forms the thread by applying pressure to the workpiece; that is, they operate by means of an external thread or thread profile that passes spirally or helically along the circumference of the tool and represents a counterpart to the shape of the screw thread that is to be created.
As a rule, a thread-grooving tool or thread-rolling tap has an approximately polygonal cross section, so that distributed along the external thread, which thus departs somewhat from a precisely cylindrical screw outline, and offset from one another there are additional outwardly projecting and in general rounded polygon corner regions, for which various terms can be used (e.g., pressure lugs, grooving teeth, forming teeth, forming wedges). These structures conform to the pitch of the helical thread profile. This configuration reduces the contact area, and hence the clamping forces, involved in in the thread-grooving process. The polygon in this case must have at least three corners or corner regions, because while in operation the thread-grooving tool or thread-rolling tap needs to be supported against the edge of the bore or the outer edge of the screw thread.
The outside diameter or cross section of the operating region of a thread-grooving tool or thread-rolling tap increases progressively, usually conically, from the front end of the tool throughout a starting or entrance region, which is the actual forming region, and then remains substantially constant in an adjacent calibration and/or guide region. As a result, the thread is created in a stepwise manner, as the teeth in the entrance region are pressed progressively deeper into the workpiece, and subsequently, if necessary, is smoothed or calibrated by the grooving teeth in the calibration region, which are all pressed in to the same depth.
In order to create a thread in a previously prepared bore, the operating region of the thread-grooving tool is inserted into the bore by being pushed forward in a linear movement axial with respect to the tool axis, i.e. the long axis of the tool shaft, while the tool is rotated about this tool axis. In this process the teeth or forming wedges or pressure lugs of the tool is thread are pressed into the surface of the workpiece, i.e. of the bore. The material of which the workpiece is made is pushed away, predominantly in the radial direction, i.e. perpendicular to the long axis of the bore. One part of the material thus deformed is made more compact, while another part is pushed into the depressions or channels between the forming wedges or teeth of the tool, which ultimately creates a screw thread in the workpiece.
In the following, these known thread-grooving tools or thread-rolling taps will also be termed “axial thread formers” and the corresponding procedure, “axial thread forming”. Known exemplary embodiments of such axial thread formers can also be found in DE 101 36 293 A1, DE 199 58 827 A1, or DE 39 34 621 C2.
The document WO 02/094491 A1 discloses a non-cutting thread-forming tool and a method for creating a screw thread without removing material, based on an operational principle that can be called circular thread forming. The thread-forming tool disclosed in WO 02/094491 A1 is elongated and comprises an operating region with one or more annular circumferential profile(s) separated from one another by annular grooves. Each circumferential profile has a polygonal shape and comprises at least three elevations or lobes as corners of a polygon, as pressure lugs. In addition, axially oriented flutes can be provided between the individual pressure lugs on the outer surface of the tool, through which coolant fluid can flow.
In the method according to WO 02/094491 A1 this tool, while being rotated about its own axis, is inserted into a bore with diameter larger than that of the tool, where it makes a circular movement along the circumference of the bore and simultaneously a forward movement into the bore, in the process forming the screw thread in the bore with no removal of material.
The thread that results according to WO 02/094491 A1, in contrast to one made by an axial thread former, is thus not produced by means of an effective surface on the tool that has a spiral shape and is adapted to the thread pitch, during a forward movement of the tool relative to the workpiece that is solely axial or linear. Instead, it is produced on one hand by effective surfaces on the tool that are annular and hence have no pitch and are polygonal in cross section, and on the other hand by a helicoid movement of the tool, which results from a linear forward movement axial with respect to the long axis of the tool and a circular movement of the long axis of the tool about a central axis of the bore, combined with rotation about its own long axis. In the following this known thread former will also be called a circular thread former or, by extension of the customary nomenclature to date, a circular thread-grooving tool, and the associated method will be called circular thread forming or circular thread-grooving.
Furthermore, combination tools are known that act as both drill and thread milling cutter, operating exclusively by the removal of material. These tools, which are called thread drill and mill cutters (Bohrgewindefraeser=BGF) (cf. EMUGE-Handbuch, Chapter 10, page 354) and circular thread drill and mill cutters (Zirkularbohrgewindefraeser=ZBGF) (cf. EMUGE-Handbuch, Chapter 10, page 355), can be used to create the bore for the thread and the thread itself in the workpiece.
The BGF has a drill part at its front end, ahead of a milling-cutter section, so that as the drill part is moved axially forward, with respect to the tool axis, while rotatiing about the tool axis it drills out a bore in the previously intact material of the workpiece, and subsequently the milling cutter, by being displaced radially further outward and making a helical backwards movement, circularly creates an internal thread in the bore.
The ZBGF likewise comprises a drill part axially ahead of a thread-milling cutter, at the front end of the tool, but in contrast to the BGF the drill part here must be smaller in its radial dimensions than the milling-cutter teeth. With a single helical movement of the ZBGF into the previously intact material of the workpiece, while rotating about its tool axis, the ZBGF simultaneously produces the bore and the internal thread within the bore.
Finally, combination threading tools with a thread tapper and a thread-groover that operate entirely axially are also known, as are associated operating procedures for producing internal threads in previously created bores. In these combination tools a thread tapper and thread-groover are disposed on a tool shaft one behind the other, i.e. are axially offset along the tool axis. When the combination tool is pushed forward axially, along its tool axis, into the bore previously created in the workpiece while the tool is rotated about its tool axis with a rotational velocity that depends on the forward velocity so as to correspond to the desired thread pitch, the thread tapper does the initial cutting of the thread and the thread-groover completes the groove formed by the initially cut thread, in a single-step operation. Thread tapper and thread-groover are continuously engaged with the workpiece during the machining movement. Such an axial combination tool and an associated method are known from DE 70 17 590 U and DE 196 49 190 C2.
According to DE 196 49 190 C2 the material is first cut away with the thread tapper to produce a thread having flanks of precise profile and size, after which the following thread-groover is applied only to the base of this previously cut thread, so as to compress it to a predetermined final diameter by deformation, with no further removal of material. As a result, especially the first turns of the thread following the initial cut are made more resistant to vibration and less susceptible to breakage, in particular for threads to be used for fixation on gray-cast-iron housings of internal combustion machines.
The shaft of a threading tool of the kind cited above is, as a rule, usually shaped so as to be at least approximately cylindrical about its long axis, and/or is received by and clamped within the chuck of a tool machine by way of its end that is directed away from the workpiece.